How many unpaired electrons are in an atom of Co in its ground state ? A) 7 B) 1 C) 3 D) 2

1 Answer
Apr 16, 2015

The answer is C) 3.

The ground state electron configuration is #"[Ar]3d"^7"4"^2#. The 3d sublevel has five orbitals, each of which can contain two electrons with opposite spin (Pauli exclusion principle) for a total of 10.

However, when adding electrons to the orbitals within a sublevel, one electron will be added to each orbital, and each with the same spin (Hund's rule), after which additional electrons will be added with a maximum of two, each having an opposite spin. (Pauli exclusion principle).

So if we look at the ground state electron configuration of cobalt, we see that the #"3d"# sublevel is not full. There are two d orbitals having paired electrons with opposite spin, and three d orbitals having one unpaired electron, each with the same spin.
http://galleryhip.com/electron-configuration-diagram.html